Reliable sequential effects accompany discrimination learning by animals during operant conditioning. These effects seemed to be controlled by the same variables that control sequential effects in human psychophysics. Local changes in stimulus range are intimately related to sequential effects, and seem particularly important in determining discrimination performance. Models of discrimination learing which make predictions solely on the basis of the physical properties of stimuli, and models which rely upon measures averaged over trials or stimuli, are not satisfactory for reliably predicting what will occur on any particular occasion. Experimental analysis of similarities and differences between range effects in operant and psychophysical settings should help clarify understanding of the significance of the dynamics of discrimination learning, and should lead to models of discrimination learning and categorization which account for performance in diverse settings.